Method and apparatus for controlling braking pressure

ABSTRACT

A method and apparatus for controlling the braking pressure in an anti-lock brake system which includes a brake power booster operable by an electric control element, wherein the rotational speed of the individual vehicle wheels is determined, the actual rotational speed value measured on a wheel during braking is compared with a memorized desired rotational speed nominal value. The electric control element is actuated as a function of the comparison result.

This application is the U.S. national-phase application of PCTInternational Application No. PCT/EP95/01860.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for controllingthe braking pressure in an automotive vehicle brake system equipped withan ABS system and including a brake power booster operable by anelectric control element, wherein the rotational speed of the individualvehicle wheels is determined.

A control method of this type is disclosed in German patent applicationNo. 42 17 409, for example. In the implementation of the methoddescribed in the application, the braking pressure introduced into thewheel brake is compared with a memorized braking pressure nominal value,and the brake power booster is operated in order to vary the boostingforce as a function of the comparison result. A disadvantage of thisknown method is the high costs incurred by the pressure measurement.

German patent application No. 42 38 333 discloses a vacuum brake powerbooster including a control valve which is electrically operable by anelectromagnet. The electromagnet interacting with the valve member ofthe control valve is rigidly connected to a valve piston which operatesthe control valve mechanically. The valve member defines a pneumaticchamber in a control housing accommodating the control valve. Thepneumatic chamber is connected to the ventilatable working chamber ofthe brake power booster by way of passages. Thus, the valve member ispermanently pressure-balanced.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a method ofbraking pressure control of the type previously referred to whichpermits achieving an accurate analog control of the pressure introducedinto the wheel brakes of the vehicle by making use of data which arealready provided in up-to-date anti-lock brake systems.

According to the present invention, this object is achieved by acomparison of the actual rotational speed value measured on the wheelduring braking with a memorized, desired rotational speed nominal valueand by operation of the electric control element as a function of thecomparison result. In specifying the present invention, the brake powerbooster is configured as a vacuum brake power booster which is operableby an electromagnet independently of the driver's wish, and the currentwhich is to be supplied to the electromagnet is controlled directlyproportionally as a function of the comparison result. Preferably, avalue of the hydraulic pressure introduced in the master brake cylinderis assigned to each value of the independent actuating force generatedby the electromagnet, to such effect that an increase in the hydraulicpressure corresponds to an increase in the independent actuating force.

In another preferred aspect of the present invention, the quality of theproposed control operation is enhanced because the current to besupplied to the electromagnet is controlled proportionally to thepneumatic differential pressure acting on the valve member. It isparticularly favorable that a pneumatic effective surface is providedwhich interacts with the valve member of the control valve and, whenacted upon by the pneumatic differential pressure, generates a forcethat counteracts the actuating force generated by the electromagnet.

The force counteracting the actuating force generated by theelectromagnet includes the force of a valve spring, which biases thevalve member in the closing direction of the control valve, and twopneumatic force components which act in opposition to each other. Thefirst force component, which counteracts the force of the valve spring,is produced by the pneumatic pressure difference between the atmosphericpressure and the pressure prevailing in the working chamber, and by afirst effective surface provided on the valve member. The second forcecomponent supporting the force of the valve spring is produced by thepneumatic pressure difference between the pressure prevailing in theworking chamber and the vacuum prevailing in the vacuum chamber, and bya second effective surface provided on the valve member.

In still another preferred aspect of the present invention, the controloperation is precisely adapted to the pneumatic ratios prevailing insidethe vacuum brake power booster because the vacuum prevailing in thehousing of the vacuum brake power booster is measured and the current tobe supplied to the electromagnet is controlled in response to the vacuumvalue determined.

According to still further aspects of the present invention, the desiredrotational speed nominal value is determined as a function of theactuating travel or the actuating speed of a brake pedal whichmechanically actuates the vacuum brake power booster, under certaincircumstances, as a function of the actuating force applied to the brakepedal. Other possibilities of determination include signaling devicesarranged outside or inside the vehicle, for example, traffic monitoringsatellites, radar sensors, or distance sensors. It is particularlyappropriate in the proposed method to measure the on-time of theelectromagnet and to take the measured value into account in theassessment of temperature effects.

The method according to the present invention will be explained in moredetail in the following text by way of an example, making reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing the operation of a brake system in whichthe method of braking pressure control of the present invention can beachieved.

FIG. 2 is an enlarged view of a control valve of a vacuum brake powerbooster which can be used in the brake system of FIG. 1.

FIG. 3 is a diagram showing the relation between the braking pressure tobe controlled and the current p=f (I) to be supplied to theelectromagnet.

DETAILED DESCRIPTION OF THE INVENTION

For implementing the control method according to the present invention,as shown in FIG. 1, the brake system for automotive vehicles mainlyincludes an actuating unit 1, an electronic control unit 8, wheel brakes12, 13, 14, 15, an ABS/hydraulic unit or pressure modulator 9 interposedbetween the wheel brakes 12 to 15 and the actuating unit 1 as well as anABS/TCS control unit (not shown) which produces control signals for thepressure modulator 9.

The actuating unit 1, in turn, includes a pneumatic brake power booster,preferably a vacuum brake power booster 2, which is operable by a brakepedal 4. A master brake cylinder 3, preferably a tandem master cylinder,is connected downstream of the brake booster 2 and has its pressurechambers (not shown) connected to the pressure modulator 9 by way ofhydraulic lines 45, 46. An actuating rod 21 is coupled to the brakepedal 4 to permit actuation of a control valve 5 (shown schematically).Control valve 5 controls the build-up of a pneumatic differentialpressure in the housing of the vacuum brake power booster 2.

A brake light switch 6 which is operatively connected to the brake pedal4 permits identifying an actuation initiated by application of the brakepedal 4. The actuating speed is sensed by a sensor device which ispreferably provided by an analog operation travel sensor 11 which isassociated with the vacuum brake power booster 2.

The travel sensor 11 which senses movements of a movable wall (notshown) generating the boosting force and which is preferably designed asa linear potentiometer, continuously produces analog output signalswhich are subjected to a time-differentiating processing operation andcorrespond respectively to an instantaneous actuating speed of the brakepedal 4. Assigned to each actuating speed is a nominal wheel speed valueω_(RSoll) memorized in a memory block 20 to be conducted to a comparatorcircuit 7 interacting with the electronic control unit 8. The nominalvalue ω_(RSoll) corresponds to a desired rotational wheel speedpreselected by the driver by depression of the brake pedal 4. Assignedto each of the vehicle wheels (not shown) is a wheel sensor 16, 17, 18,19 whose signal corresponding to an actual rotational wheel speed valueω_(RIst) is compared to the desired wheel speed nominal value ω_(RSoll)in the comparator circuit 7. The electronic control unit 8 generatescontrol signals to adjust the vacuum brake power booster 2 as a functionof the comparison result produced in the comparator circuit 7.Independent actuation of the control valve 5 of the vacuum brake powerbooster 2 is effected by way of an electromagnet 22 (FIG. 2) operable bythe control signals of the electronic control unit 8, irrespective ofthe actuating force introduced on the brake pedal 4.

As can be seen in FIG. 2 in particular, the control valve 5 isaccommodated in a control housing 23 which is sealed and guided in thehousing of the vacuum brake power booster 2 shown in FIG. 1. Controlvalve 5 includes a first sealing seat 24 provided on the control housing23, a second sealing seat 26 provided on a valve piston 25 connected tothe actuating rod 21 and a valve member 27 interacting with both sealingseats 24, 26. Further, a laterally extending channel 28 is provided inthe control housing 23 and permits a connection between the workingchambers of the vacuum brake power booster 2 (not shown).

As can further be seen in FIG. 2, the valve member 27 has an annularsealing surface 31 interacting with the two sealing seats 24, 26.Sealing surface 31 has several axial passages 33 and is reinforced bymeans of a metallic reinforcing disc 32. Radially inwardly adjoining thesealing surface 31 is a sealing lip 34 which, in the mounted conditionof the valve member 27 in the control housing 23, is in sealing abutmenton the housing inside wall or on a retaining ring 35 retaining the valvemember 27 to define a pneumatic chamber 36 in the control housing 23.The flow ducts provided by the passages 33 (not referred to in detail)connect the pneumatic chamber 36 to an annular chamber 37 confined bythe sealing seats 24, 26. A pneumatic channel 38 connected to theventilatable working chamber terminates into annular chamber 37. Theresult is that the pneumatic chamber 36 arranged on the side of thevalve member 27 remote from the sealing surface 31 is constantlyconnected to the working chamber (not shown), and pressure balanceoccurs on the valve member 27.

The radially inward diameter of the sealing lip 34 and the secondsealing seat 26 provided on the valve piston 25 define a first pneumaticeffective surface 39 on the valve member 27. Application of thepneumatic difference in pressure between the atmospheric pressure andthe pressure prevailing in the working chamber to the effective surface39 results in a first force component. The first force componentcounteracts the force of a valve spring 40 which preloads the valvemember 27 in the closing direction of the control valve 5 and is reducedwith increasing ventilation of the working chamber or chamber 36. Theapplication of the pneumatic difference in pressure between the pressureprevailing in the working chamber and the vacuum prevailing in thevacuum chamber to a second pneumatic effective surface 41, provided inthe intermediate area of the valve member 27, causes a second pneumaticforce component which assists the effect of the above-mentioned valvespring 40 and increases with increasing ventilation of the workingchamber or chamber 36.

To initiate the above-mentioned operation of the brake power boosterwhich is independent of the actuating rod 21, a third sealing seat 29 isarranged radially between the first sealing seat 24 and the secondsealing seat 26. Sealing seat 29 is operable by way of the electromagnet22 which is arranged in an axial bowl-shaped extension 30 of the valvepiston 25 and, consequently, is displaceable along with the valve piston25 in the control housing 23.

The electromagnet 22 has a coil 47, which is slipped onto a guideelement 44 attached within the extension 30, and a cylindrical armature42 which is slidable in the coil. Armature 42 is in force-transmittingconnection with a sleeve 43 sealed in the control housing 23. Sleeve 43includes the third sealing seat 29 permitting the actuating forcegenerated by the electromagnet 22 to be transmitted to the third sealingseat 29. In this arrangement, the third sealing seat 29 is axiallyoffset (see distance `b`) with respect to the second sealing seat 26provided on the valve piston 25.

During an independent braking operation initiated by energization of thecoil 47, the armature 42 is displaced to the right, as viewed in thedrawing, causing the third sealing seat 29 to initially move intoabutment on the sealing surface 31 of the valve member 27 after havingcovered the distance `b`. This abutment bridges the first sealing seat24 provided on the control housing 23 in terms of effect so that thereis no connection between the pneumatic working chambers of the brakepower booster 2. Subsequently, the third sealing seat 29 and the valvemember 27 will move jointly, and the second sealing seat 26 is openedand the ventilatable working chamber of the vacuum brake power booster 2is ventilated. The movement of the third sealing seat 29 lasts until thearmature 42 abuts on the guide element 44 and the slot `s` between thetwo parts becomes zero. In the absence of actuating force on theactuating rod 21, the control housing 23 covers a distance relative tothe valve piston 25 which corresponds to the distance `a` between across member 48 limiting the movement of the valve piston 25 and a stopsurface 49 provided on the control housing 23. The reason for thismovement is a piston rod return spring 50 which moves the valve piston25 to the right by way of the actuating rod 21 and tries to close thesecond sealing seat 26 again. However, because the third sealing seat 29is synchronously entrained due to the rigid connection between theelectromagnet 22 and the valve piston 25, the slot between the valvemember 27 and the second sealing seat 26 is kept open, namely by thedimension s-b. The ventilatable working chamber of the brake powerbooster 2 is thereby connected to the atmosphere, and brake force isgenerated.

To ensure that the electromagnet 22 is reliably disconnected upontermination of the braking operation assisted by independent force, anelectric switching device 10 is provided which is shown onlyschematically in FIG. 1.

As has already been mentioned hereinabove, the desired wheel speednominal value ω_(RSoll) assigned to the actuating speed of the brakepedal 4 and the actual wheel speed value ω_(RIst) determined on thewheel are compared in the comparator circuit 7 during controloperations. A signal responsive to the comparison result actuates theelectronic control unit 8 to generate control signals or current signalsfor the electromagnet 22 which actuates the control valve 5 of thevacuum brake power booster 2. It must be ensured that the independentactuating force generated by the electromagnet 22 is in excess of thepreviously described sum of the force components acting on the valvemember 27 of the control valve 5. It can be taken from the descriptionof the operation of control valve 5 illustrated in FIG. 2 that anincrease of the independent actuating force generated by theelectromagnet 22 or the current supplied to the electromagnet 22 causesan increase in the hydraulic pressure generated by the vacuum brakepower booster 2 in the master brake cylinder 3. FIG. 3 of the drawingshows the relation between the hydraulic pressure generated in themaster brake cylinder 3 and the electromagnetic independent actuatingforce or the electric current to be supplied to the electromagnet 22.

If the comparison between the wheel speed nominal value ω_(RSoll)assigned to the pedal application speed and the actual wheel speed valueω_(RIst) determined by one of the sensors 16 to 19 shows that thenominal value ω_(RSoll) is smaller than the actual value, the controlvalve 5 of the vacuum brake power booster 2 is actuated with the effectof a pressure increase in the master brake cylinder 3 until thedifference between both rotational speed values becomes zero. Thepressure introduced into the master brake cylinder 3 can be maintainedby energization of the electromagnet 22 with a constant current.Pressure reduction in the master brake cylinder 3 during the controloperation is achieved by correspondingly reducing the current suppliedto the electromagnet 22. Should, however, the comparison show that thenominal value exceeds the actual value, the excessive braking pressureintroduced into the wheel brakes may be reduced either by interventionof the existing ABS system or by reduction of the current supplied tothe electromagnet 22.

We claim:
 1. Method of controlling the braking pressure in an automotivevehicle brake system equipped with an ABS system and including a vacuumbrake power booster operable by an electromagnet independently of thedriver's wish, and a master brake cylinder connected downstream of thevacuum brake power booster, said method comprising the steps of:(a)determining a rotational speed of individual wheels of a vehicle; (b)assigning a value of hydraulic pressure introduced in the master brakecylinder to a value of an independent actuating force generated by theelectromagnet based on the rotational speed of individual wheels of thevehicle; and (c) controlling a current supplied to the electromagnet toincrease the independent actuating force generated by the electromagnetwhen the hydraulic pressure in the master brake cylinder is increasedbased on the assignment of step (b).
 2. Method as claimed in claim 1,wherein the electromagnet actuates a valve member of a control valvewhich controls a pneumatic differential pressure acting in the vacuumbrake power booster, characterized in that the current supplied to theelectromagnet is controlled proportionally to the pneumatic differentialpressure acting on the valve member.
 3. Method as claimed in claim 2,wherein the electromagnet actuates the valve member of the control valvewhich controls the pneumatic differential pressure acting in the brakepower booster, and wherein the master brake cylinder is connecteddownstream of the brake power booster, characterized in that a value ofthe hydraulic pressure introduced in the master brake cylinder isassigned to each value of the independent actuating force generated bythe electromagnet, to such effect that an increase in the hydraulicpressure corresponds to an increase in the independent actuating force.4. Method as claimed in claim 3, characterized in that the currentsupplied to the electromagnet is controlled proportionally to thepneumatic differential pressure acting on the valve member, as afunction of a comparison between the rotational speeds of individualwheels of the vehicle and a desired rotational speed nominal value. 5.Method as claimed in claim 4, characterized in that a pneumaticeffective surface interacting with the valve member of the control valveis provided and, when acted upon by the pneumatic differential pressure,generates a force that counteracts the actuating force generated by theelectromagnet.
 6. Method as claimed in claim 5, wherein the vacuum brakepower booster includes an evacuatable vacuum chamber and a ventilatableworking chamber, characterized in that the force counteracting theindependent actuating force generated by the electromagnet includes theforce of a valve spring, which preloads the valve member in the closingdirection of the control valve, and two pneumatic force components whichact in opposition to each other, wherein the first force component,which counteracts the force of the valve spring, is produced by thepneumatic pressure difference between the atmospheric pressure and thepressure prevailing in the working chamber, and by a first effectivesurface provided on the valve member, and the second force componentsupporting the force of the valve spring is produced by the pneumaticpressure difference between the pressure prevailing in the workingchamber and the vacuum prevailing in the vacuum chamber, and by a secondeffective surface provided on the valve member.
 7. Method as claimed inclaim 6, characterized in that the vacuum prevailing in the housing ofthe vacuum brake power booster is measured, and the current supplied tothe electromagnet is controlled in response to the measured vacuumvalue.
 8. Method as claimed in claim 7, wherein the vacuum brake powerbooster is operable by a brake pedal, characterized in that an actuatingspeed of the brake pedal is sensed, and the desired rotational speednominal value is determined as a function of the sensed actuating speedof the brake pedal.
 9. Method as claimed in claim 8, characterized inthat an actuating travel of the brake pedal is sensed, and the desiredrotational speed nominal value is determined as a function of the sensedactuating travel of the brake pedal.
 10. Method as claimed in claim 8,characterized in that an actuating force acting upon the brake pedal issensed, and the desired rotational speed nominal value is determined asa function of the sensed actuating force acting upon the brake pedal.11. Method as claimed in claim 8, characterized in that the desiredrotational speed nominal value is determined as a function of an outputsignal of a signaling device arranged at a location outside the vehicle.12. Method as claimed in claim 11, wherein the signaling device is atraffic monitoring satellite.
 13. Method as claimed in claim 8,characterized in that the desired rotational speed nominal value isdetermined as a function of an output signal of a signaling devicearranged at a location inside the vehicle.
 14. Method as claimed inclaim 13, wherein the signaling device is a distance sensor.
 15. Methodas claimed in claim 8, characterized in that an on-time value of theelectromagnet is determined and taken into account in an assessment oftemperature effects.
 16. Apparatus for controlling the braking pressurein an automotive vehicle brake system equipped with an ABS system, saidapparatus comprising:a brake pedal; a vacuum brake power boosterresponsive to movements of said brake pedal; a master brake cylinder,responsive to said vacuum brake power booster, for controlling pressurein wheel brakes of the vehicle brake system; means for developingindications of the actual rotational speeds of wheels of the vehicle;means for sensing the actuation speeds of said brake pedal and fordeveloping indications of the actuation speeds of said brake pedal;means for storing nominal values of the rotational speeds of the wheelsof the vehicle corresponding to the speeds of actuation of said brakepedal and for selecting nominal wheel rotational speeds corresponding tothe brake pedal actuation speeds upon applications of said brake pedal;means for comparing the actual wheel rotational speeds with the selectednominal wheel rotational speeds and for developing indications of thedifferences between the actual wheel rotational speeds and the nominalwheel rotational speeds; and means, responsive to the indications ofdifferences between the actual wheel rotational speeds and the nominalwheel rotational speeds, for controlling said vacuum brake power boosterindependent of movements of said brake pedal.
 17. Apparatus according toclaim 16 wherein said means for controlling said vacuum brake powerbooster independent of movements of said brake pedal include:(a) anelectronic control unit, responsive to said comparison means, fordeveloping control signals representative of differences between theactual wheel rotational speeds and the nominal wheel rotational speeds,(b) an electromagnet responsive to the control signals, and (c) acontrol valve, responsive to said electromagnet, for controlling saidvacuum brake booster.
 18. Apparatus according to claim 17 wherein saidcontrol valve controls the pneumatic differential pressure acting insaid brake power booster.
 19. Apparatus according to claim 18 whereinsaid control valve has a valve member which is actuated by saidelectromagnet to control the pneumatic differential pressure acting insaid brake power booster in response to the control signals.
 20. Amethod of controlling the braking pressure in a brake system including avacuum brake power booster operable by an electromagnet, said methodcomprising the steps of:(a) measuring a rotational wheel speed; (b)sensing an actuation speed of a brake pedal; (c) assigning a nominalwheel speed to the actuation speed of the brake pedal; (d) comparing therotational wheel speed to the nominal wheel speed; and (e) controlling acurrent supplied to the electromagnet based on the result of thecomparison of step (d).